How to use nose's assert_raises?

python nosetests
user1544624 picture user1544624 · Aug 1, 2012 · Viewed 31.6k times · Source

I've searched for documentation, but couldn't find any. There were a couple that didn't explain much.

Can someone explain to me Nose's 

assert_raises(what should I put here?)

function and how to use it?

Answer

BuddhiP picture BuddhiP · Oct 11, 2013

While the accepted answer is correct, I think there is a better use to assert_raises method.

If you simply want to assert that an exception occurs, it's probably simpler and cleaner to use @raises syntax.

@raises(HTTPError)
def test_exception_is_raised:
    call_your_method(p1, p2)

However, assume you want to do bit more with the raised exception, for example: we need to assert that raised HTTPError is of type 401: Unauthorized, instead of 500: Server Error.

In such a situation above syntax is not that helpful, we should use the assert_raises but in a different way. If we do not pass it a callable as the second parameter assert_raises will return back a context which we can use to further test the exception details.

def test_exception_is_raised:
    with assert_raises(HTTPError) as cm:
         call_your_method(p1, p2)
    ex = cm.exception # raised exception is available through exception property of context
    ok_(ex.code == 401, 'HTTPError should be Unauthorized!')

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